Fixed versus Variable Costs

2 minute read

Fixed and variable costs are basic concepts in business and energy engineering. Unfortunately this doesn’t stop experienced energy professionals from misuing them.

I once worked on a project looking at the feasibility of a Combined Heat & Power (CHP) plant at a pharmaceuticals site. Previously two consultants had looked at the site and concluded that a 2 MWe gas engine would be an economic project. We expected that good CHP projects would have payback periods of five to six years. These two consultants had modelled a payback below two years!

We took a look at the technical fundamentals (annual profiles of site heat & power demands) and agreed that the site was well suited to a 2 MWe gas engine. However once we took a step further and started to evaluate the economics things started to unravel.

A crucial part of any energy project are prices – for CHP gas and electricity prices are crucial. One way to model electricity prices is to use historic prices for the site based on the last twelve months of electricity invoices.

What we wanted to know was the variable electricity price. This price will represent the savings for reducing import electricity to site through use of on-site generation. Table 1 below shows a simplified electricity invoice.

Table 1 – A simplified electricity invoice    
Electricity consumed MWh 2,500
Variable electricity price £/MWh 65
Variable electricity cost £ 162,500
Fixed costs £ 100,000
Total annual cost £ 262,500
Delivered electricity cost £/MWh 105

When we compared our analysis of the electricity invoices to the prices used by the two consultants, we were shocked to find they had used they delivered electricity cost in their model – not the variable cost. This overstated the value of electricity saved and therefore value of the CHP project.

To understand why this is a problem think what would happen if we saved all 2,500 MWh of electricity. This would mean we have no variable cost – a saving of £162,500. But even with no electricity consumed the site would still pay fixed charges of £100,000.

However if we use a value of £105/MWh , we calculate a saving of £262,500 – which is incorrect! While this seems obvious when spelt out, two consultants from separate well renowned energy companies made this mistake.

It was difficult going back to the pharmaceuticals site and telling them their fantastic project was perhaps not so fantastic. But it is far better to understand it now rather than after the engine was installed.

You might be wondering why people calculate a delivered electricity cost at all. It can be useful for understanding the total cost for delivering electricity to a site. But it is not useful for understanding the savings from projects like CHP or reducing site electricity consumption.

One lesson is to understand that not all electricity prices are equivalent. When you are given an electricity price be sure that you understand what that price represents.

Another lesson here is to always check your customer’s actual invoices. It takes time but it is worth it to build up an accurate picture of what is going on by replicating the invoice from the rates (£/MWh) and amounts (MWh). What you want to arrive at is a total marginal and a total fixed cost.

Thanks for reading!

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